Wadi Yalamlam is
known as one of the significant wadis in the west of Saudi Arabia. It is a
very important water source for the western region of the country. Thus, it
supplies the holy places in Mecca and the surrounding areas with drinking
water. The floristic composition of Wadi Yalamlam has not been
comprehensively studied. For that reason, this work aimed to assess the wadi
vegetation cover, life-form presence, chorotype, diversity, and community
structure using temporal remote sensing data. Temporal datasets spanning
4 years were acquired from the Landsat 8 sensor in 2013 as an early
acquisition and in 2017 as a late acquisition to estimate normalized
difference vegetation index (NDVI) changes. The wadi was divided into seven
stands. Stands 7, 1, and 3 were the richest with the highest Shannon index
values of 2.98, 2.69, and 2.64, respectively. On the other hand, stand 6 has
the least plant biodiversity with a Shannon index of 1.8. The study also
revealed the presence of 48 different plant species belonging to 24 families.
Fabaceae (17 %) and Poaceae (13 %) were the main families that form
most of the vegetation in the study area, while many families were
represented by only 2 % of the vegetation of the wadi. NDVI analysis
showed that the wadi suffers from various types of degradation of the
vegetation cover along with the wadi main stream.

The Kingdom of Saudi Arabia is a big desert with a land area of
approximately 2 250 000 km2 comprising the basic area of the Arabian
Peninsula. Based on that, xerophytic vegetation forms the distinguished
topographies of the plant life in the country (Khalik et al., 2013).
According to Abuzinada et al. (2005), the natural areas and biological
diversity are very large in the kingdom, and these factors are very important
for dealing with ecosystems. The vegetation structure in Saudi Arabia
presents differences in a distributional manner that arise from
changes in different factors and resources such as weather and soil
variables, anthropogenic pressures, and water (Hegazy et al., 2007).

The geographical location of Saudi Arabia between the surrounding continents
indicates the importance of the vegetation structure in the kingdom. Hence,
the flora contains different global elements such as the Palaearctic (located
in Asia and Europe), Afrotropical (located in Africa), and the Malayan–Indo
world (Ghazanfar, 2006). Saudi Arabia has three categories of plant
diversity: Sudano–Deccanian, Saharo–Sindian, and tropical Indian–African
(Alfarhan, 1999; Thomas et al., 2008). According to Collenette (1998), some
areas in Saudi Arabia like the Asir, Alhejaz, and the western mountains have
high floristic diversity. These mountain chains are near the Red Sea and they
have the greatest level of rainfall (Şen et al., 2017). The height of
these mountains reaches up to 2850 m. Some researchers have demonstrated
that the topography and climate of the area are affecting the level of
speciation (Abulfatih, 1992; El-Kady et al., 1995; Shaltout and Mady, 1996;
Shaltout et al., 1997). The flora of Saudi Arabia is reasonably well
identified at the taxonomic level. The species richness of the 15 protected
areas controlled by the National Commission for Wildlife Conservation and
Development, as well as many of the zones protected by the administration of
the Ministry of Agriculture, is somehow well documented in the work of Forbis
et al. (2006), but this was more than 10 years ago. The number of verified
species in Saudi Arabia is growing day by day based on recent field trips and
biodiversity studies. An example is that over 1500 species were recorded by
Migahid (1978) between the years 1974 and 1988. This number was raised to
2300 within a period of about 3 decades according to the accounts given in
the Flora of Saudi Arabia (Chaudhary, 1999, 2000; Alfarhan et al., 2005; Masrahi et
al., 2012). Several scholarly works were conducted on the flora of Saudi
Arabia. Two of the most comprehensive works on the flora of Saudi Arabia were
conducted by Migahid (1978) and Chaudhary (1999, 2000). There are some
studies on different areas of Saudi Arabia such as Schulz and Whitney (1986),
who studied the vegetation and floras of the sabkhas, hillocks, and other
prominent mountains of the Najd regions Twaik, Aja, and Salma. Considerable
efforts have also been made toward the elucidation of
vegetation–environmental relationships in the ecosystems of “raudhas” or
depressions (Shaltout and Mady, 1996; Sharaf El Din et al., 1999; Alfarhan,
2001). The plant communities of wadis have been recorded in some studies like
Wadi Al-Ammaria by Al-Yemeni (2001) and Wadi Hanifa by Taia and El-Ghanem
(2001) and El-Ghanem (2006). But no previous study has been done on the flora
of Wadi Yalamlam.

The realization of the normalized difference vegetation index (NDVI) is a robust
spectral index using the near-infrared (NIR) and red bands of both satellite and aerial
multispectral imagery computed across scene pixels in time and
space. The NDVI has been successful in monitoring and assessing vegetative cover
as well as vegetation conditions (Van Leeuwen, 2008; Elhag, 2016a). The main
purpose of NDVI applications is to distinguish between healthy and stressed
vegetation in addition to forest type discrimination (Lambin, 1994; Rindfuss
and Stern, 1998).

Therefore, the aim of the current research is to study the vegetation
cover in Wadi Yalamlam from different aspects, such as species richness, life-form presence, and biodiversity in relation to
habitat change in the study area.
The normalized difference vegetation index has been conducted from temporal
remote sensing data to assess the status of the vegetation cover within the
designated study area over the last 4 years. Moreover, species diversity
indices have been used to discriminate vegetation sets and to evaluate the
relation between the vegetation aspects in the study area.

2.1 Study area

The location of Wadi Yalamlam is about 100 km south of Mecca city between
20∘26′, 21∘8′ N and
39∘45′, 40∘29′ E (Fig. 1). The wadi basin covers a
large area of about 180 000 ha. The border of the basin located in the
downstream area is expanded to comprise almost all the flat area in the
lower part. Wadi Yalamlam initiates from the high altitudes of the Hijaz Mountains
near Taif from the Al Shafa area. Its average annual rainfall is
ca. 140 mm. The wadi has different altitudes greatly varying from 2850 to
25 m (a.s.l.) in the upstream and downstream areas, respectively. The main route
of Wadi Yalamlam is traversed by greatly cracked granitoid, gabbroic, and
metamorphic rocks until it reaches the Red Sea coastal plain and it is about
120 km in length. Incisive natural vegetation covers the higher and the
central parts of the basin. On the other hand, Quaternary deposits and sand
dunes accompanied by tiny scattered particles vastly alter the granitoid and
metamorphosed basaltic hills, which are the constitutes of the lower part of
the wadi. Several basic ditches are observed in the lower part of the basin.
Moreover, the depth of the Quaternary deposits of the wadi is larger in the
lower part.

2.2 Climate of the study area

The climate of the Red Sea coast is usually stable as the weather is cold in
the winter season and warm in the summer. Based on the weather recorded, the
average maximum temperature is between 37 and 39 ∘C, and the minimum
temperature is around 19 ∘C. The highest temperature was
49 ∘C and the lowest was 12 ∘C. The maximum average
evaporation value is between 450 and 550 mm in summer, while in winter it is
around 200 mm (Subyani and Bayumi, 2003).

2.3 Sample sites

Samples were chosen along Wadi Yalamlam areas such as (Fig. 2)

upstream and midstream,

downstream parts,

and different wadi streams.

The study area was visited from the beginning of March 2015 to the end of
February 2016. Seven stands were randomly chosen in every area for the
current investigation during different growing seasons. The random selection
of stands was carried out according to de Vries (1986) with the stratified
random sampling technique.

Locations and samples were selected as an example of a large range of
physiographic and environmental variability in every branch.

Sample plots were randomly selected using the relevé process in every
site described by Mueller-Dombois and Ellenberg (1974).

The plots were 10×10 m and samples were taken through the spring
season when taxa were expected to be growing and flowering. The vegetation
sampling included recording all plant taxa in the plots.

The plant cover of each taxon was estimated using the Zurich–Montpellier
technique (Braun-Blanquet et al., 1965). The collected sample specimens were
recognized according to Collenette (1999), Cope (1985), Rahman et al. (2004),
and Chaudhary (1999, 2000).

2.4 Realization of species richness equations

Various indices have been developed for examining species richness in a
region based on estimations of the relative abundance of the species
derived from samples (Heip et al., 1998). Among these indices are the
Shannon–Wiener information function (Lloyd et al., 1968), the Simpson's
dominance index (Hunter and Gaston, 1988), the Margalef species richness
index (Meurant, 2012), and the Pielou evenness index (Pielou, 1966). The first
two were used in the current study due to the linkage between a common family
of diversity indices and nonadditive statistical mechanics (Keylock, 2005).

2.4.1 The Shannon index

The main principle of this index is that the diversity of a community is the
amount of data in a code. It is calculated as follows.

(1)H=-∑i=1Spi×ln⁡pi=-∑i=1SniN×ln⁡niN

In this formula, S is the total number of species, N is the total number
of individuals, and ni is the number of individuals of the ith species.
niN is equivalent to pi, the probability of finding the ith
species.

2.4.2 Simpson's index

Simpson's approach for assessing species diversity evaluates the dominance of
a species relative to the number of species in a sample or population (Hunter
and Gaston, 1988). It is calculated as follows.

(2)D=Σnini-1/NN-1

D is the Simpson diversity index, ni is the number of individuals
belonging to i species, and N is the total number of individuals.

2.5 Density analysis

Predictive vegetation modeling is one of the commonly used
methods. It is described as “predicting the distribution of vegetation
across a landscape based on the relationship between the spatial distribution
of vegetation and certain environmental variables” (Franklin, 1995; Guisan
and Zimmermann, 2000). Concepts of spatial variations are obtained according
to the following equations.

(3)γk=12nk⋅∑i=12kzxi-zxi+k2,

where n(k) is the number of pairs of observation, and Z(xi) is the
feature property measured in point x and in point x+k.

(4)Z⋅x0=∑i=1nλi⋅zxi,

where Z⋅(x0) is the interpolated value of variable Z at location,
x0, Z(xi) represents the values measured at location xi, and
λi is the weighed coefficient calculated based on the
semivariogram when

∑i=1nλi=1.

Consequently, it is possible to obtain non-biased interpolated values; that
is, the expected value EZ⋅(x0)-Z(x0)=0 and the
estimated variance Var.Z⋅(x0)-Z(x0)= minimum
(Elhag and Bahrawi, 2016).

The relationship between environment and vegetation could be associated with
the observed connection or the hypothetical or investigational
physiological limitations of diverse plant taxa. This relationship has been
calculated using statistical methods. These statistical methods have gradually become
more flexible to show what is known as a non-Gaussian species
response curve (Heath and Smith, 1989).

2.6 NDVI change detection

The multispectral remote sensing data image was obtained from the United
States Geological Survey (USGS). Landsat 8 images consist of nine spectral
bands ranging from visible to thermal infrared with a spatial resolution of
30 m for bands from 1 to 7 and then 9. The resolution for the panchromatic
band 8 is 15 m. Spectral bands are selectable across the range 435 to
1251 nm. The temporal datasets were acquired in April 2013 as an early date
of acquisition and in April 2017 as a late date of acquisition (Path, 169;
Row, 46).

Temporal datasets were preprocessed to maximize all possible reasons for data
correction. Radiometric, geometric, and atmospheric corrections were made
according to Vogelmann et al. (2001) and Elhag and Bahrawi (2017).

There are quite a few indices for defining vegetation behavior zones on a
remote sensing imagery, one of which is the NDVI (Bhandari et al., 2012). It is a
crucial and commonly used vegetation index. In addition, it is widely applied
to research works related to climatic and global environmental changes
(Bhandari et al., 2012). NDVI can be estimated as a ratio variance between
measured canopy reflectance in the red and near-infrared bands, respectively
(Elhag and Bahrawi, 2017). A schematic flowchart of the adopted methodology is
illustrated in Fig. 3.

3.1 Floristic analysis and plant diversity of the study area

Vegetation in the seven stands was represented by 48 species belonging to 24
families. The families Fabaceae and Poaceae were the richest (17 %),
(13 %) followed by Zygophyllaceae (10 %), Cucurbitaceae (10 %)
and Euphorbiaceae (6 %), Asclepiadaceae, Molluginaceae, Cleomaceae,
Solanaceae, and Caryophyllaceae (4 %), and 14 families were represented by
only (2 %) of the vegetation of the wadi (Figs. 4 and 5).

Many studies and comparisons of families involving a large number of species
have been conducted for various regions of Saudi Arabia such as the Asir Mountains in
Hosni and Hegazy (1996), Mosallam (2007) in the Taif area, Alatar et
al. (2012) in the Al-Jufair Wadi, and Al-Turki and Al-Olayan (2003) in the Hail
region. Similar studies have also been recorded outside
the kingdom like in Egypt (El-Ghani and Abdel-Khalik, 2006; El-Ghani and
El-Sawaf, 2004) and Jebel Marra in Sudan (Al-Sherif et al., 2013). The most
famous plant species in Saudi Arabia belong to the families Fabaceae and
Asteraceae (Migahid, 1978; Chaudhary, 1999; Rahman et al., 2004).
Poaceae is the largest family listed by some researchers, but there are also
other large families in the flora of Saudi Arabia (Collenette, 1999; AlNafie,
2008).

Stand 1 was the most diverse with about 28 different taxa, followed by
stand 7 with about 22 different taxa because it is surrounded by and near the
water dam. Stand 6 was the least diverse with seven taxa only.

3.2 Plant growth forms of the study area

It was observed that herbs dominated the vegetation of the study area
(48 %), followed by shrubs (19%), grass (11 %) shrubs to trees
(10 %), and subshrubs (6 %) (Fig. 6). The higher number of species
belonged to herbs, followed by grasses, shrubs, and trees. These
observations of many differences in vegetation cover composition and
structure can be attributed to inundation, competition, and the environmental
factors that might affect vegetation communities on the wadi (Lenssen et
al., 1999; Zhang et al., 2005). The difference in density, frequency, and
abundance between taxa might be attributed to variation in the habitat
(Nardi et al., 2016).

3.3 Plant life-forms in the study area

The life-form range of the study area showed a predominance of therophytes
and chamaephytes, which constituted 31 % and 29 % of the total
flora, respectively, followed by phanerophytes 19 %, while
hemicryptophytes are 17 %. Then both geophytes and epiphytes represent
2 % of the total flora as shown in Fig. 7. The life-form spectrum in the
study area is distinguished by an arid desert region with a dominance of
therophytes. This result supports the theory of Cain (1950) and Deschenes
(1969), which states that “dry climate, overgrazing, and trampling which is
so prevalent on grasslands, tend to increase the percentage of therophytes
through the introduction and spread of weedy grasses and forbs of this life-form”. Furthermore, the
high percentage of therophytes could be also
reflecting human activities as claimed by Barbero et al. (1990). Therophytes
(annuals and biennials) are not unexpectedly recorded for 60 % of the
overall taxa of the region. They generally bloom and form well-developed
growth in the wadis and at the base of steady dunes, where water gathers
after rain. Moreover, it is essential to specify that the
dominance of both Fabaceae and therophytes in local flora can be an
indicator of the relative index of disturbance for Mediterranean ecosystems
(El-Ghani and Abdel-Khalik, 2006). These results are in agreement with the
life-form scales among desert habitats in further parts of Saudi Arabia
(El-Demerdash et al., 1994; Collenette, 1999; Chaudhary, 2000; Al-Turki and
Al-Olayan, 2003; El-Ghanim et al., 2010; Alatar et al., 2012; Daur, 2012).

3.4 Species richness in the study area

The values of the Shannon index in the study area are as follows: 1.8 (stand 6),
2.20 (stand 4), up to 2.69 (stand 1), 2.64
(stand 3), and 2.98 (stand 7) (Fig. 8). Shannon index examination demonstrates
a high species diversity. Typically, the Shannon index in real ecosystems
ranges between 1.5 and 3.5 (MacDonald and MacDonald, 2003). The value rarely
surpasses 4 (Margalef, 1972).

The value of the Simpson's index ranges from 0 to 1. With this index, 0 represents
infinite diversity and 1 represents no diversity. That is, the bigger the value the
lower the diversity (Hunter and Gaston, 1988). Simpson's results in the study
area showed that the values of the index are 0.88 (stands 1, 5, and 6), 0.92
(stand 4), 0.94 (stand 3), 0.95 (stand 2), and 0.96 (stand 7) (Fig. 9). This
means that stands 1, 5, and 6 have the highest biodiversity, while the
lowest is stand 7.

3.5 Plant density mapping of the study area

The main life-forms are chamaephytes, phanerophytes, therophytes,
hemicryptophytes, geophytes, and epiphytes.

The normalized difference vegetation index was used to evaluate the status of
Wadi Yalamlam vegetation cover compared to data obtained 4 years ago
(Fig. 10a, b). NDVI thematic change detection showed a decrease in vegetation
cover (Fig. 11). Upper-stream areas of Wadi Yalamlam were the most fragile
parts of the wadi basin due to anthropogenic activities (Bahrawi et
al., 2016). The midstream section of Wadi Yalamlam showed no significant
difference in vegetation cover. Such stability in vegetation cover is
explained by the water availability in the midstream section due to its
morphometric features (Elhag et al., 2017). The vegetation cover of the lower
section of the Wadi Yalamlam basin was not abundant in either temporal dataset.
The lower section has mainly alluvial deposits occurring frequently due to
soil erosion (Elhag, 2016b; Bahrawi et al., 2016).

The current research focuses on species richness
and species diversity in the designated study area due to its local
importance as a major torrent of the holy Makkah region. The conducted field
surveys in addition to the Shannon index examination demonstrate a high species
diversity in different plant growth forms across the designated wadi system.
Moreover, the spatial configuration of the vegetation cover in Wadi Yalamlam
shows significant variation in terms of the normalized difference vegetation
and species richness indices. The temporal analysis of the
normalized difference vegetation index shows low values at the upper-stream
section of the wadi, which requires immediate regulation to stop losing the
species diversity. Consequently, restoration and rehabilitation schemes
should
be adopted in the upper-stream section of the wadi. Meanwhile, sediment
transport should be regulated in the lower-stream section to allow the natural
vegetation to succeed at the lower-stream section. As a recommendation, more
investigations should be carried out to identify threatened plant species
and to implement effective monitoring plans.

ME (King Abdulaziz University) and AA (King Abdulaziz
University) were responsible for the data analysis and wrote most of the
paper. AH (Cairo University) proposed the project idea and participated in
all the fieldwork, data analysis, and writing of the paper. HG (Assiut
University) participated in the data analysis. NM (King Abdulaziz University)
performed the data interpretation.

This project was funded by the Deanship of Scientific Research (DSR), King
Abdulaziz University, Jeddah, under grant no. G-235-247-38. The authors
therefore acknowledge and thank DSR for technical and financial
support.

Abuzinada, A. H., Al-Wetaid, Y., and Al-Basyouni, S. Z. M.: The National
Strategy for Conservation of Biodiversity in the Kingdom of Saudi Arabia, The
National Commission for Wildlife Conservation and Development, Conservation
of Biological Diversity, Riyadh, Saudi Arabia, 2005.

Barbero, M., Bonin, G., Loisel, R., and Quézel, P.: Changes and
disturbances of forest ecosystems caused by human activities in the western
part of the Mediterranean basin, Vegetatio, 87, 151–173, 1990.

The current article focuses on plant diversity assessment in arid environments. Species richness and species evenness equations were used to meet the objectives. Remote sensing techniques were used to detect normalized difference vegetation index (NDVI) temporal changes. Two datasets were used to realize the NDVI, and post-chance detection (PCC) techniques were used to evaluate plant diversity status over a period of 4 years. The results show a recognizable loss in plant biodiversity.

The current article focuses on plant diversity assessment in arid environments. Species richness...